The invention relates to a method for linearising a power amplifier and to an amplifier using such method. It relates also to a transmitter comprising a power amplifier of this kind.
Generally, power amplifiers are used near saturation in order to obtain the most efficient use of such amplifiers. However, near saturation, the amplifier has a non linear behavior, i.e. the gain decreases sharply for high input signals compared to smaller input signals; moreover the output signal is phase distorted due to the well known AM/PM conversion, i.e. due to the conversion of amplitude modulation into phase modulation.
The distortion of the output signal with respect to the input signal is generally not admissible as far as non constant envelope signals are used. This is more particularly true in the field of telecommunications which necessitates both power efficiency and good qualities of signals.
In order to linearise amplifiers, several solutions are known. However, it has been observed that none of the conventional solutions can provide power efficiency over a wide frequency band as it is often requested more particularly in the above mentioned field of telecommunications. For instance, in future radio telephone systems, use will be made of CDMA (Code Division Multiple Access) where the modulation is spread over a frequency band of several megahertz. In fact, in CDMA, to each symbol, such as a bit, is superposed a code which is a sequence at a higher frequency.
The only solution which is known, up to now, for the linearisation over a wide frequency band of a non constant envelope signal is the analog feed forward technology wherein no feed back loop is provided. It corrects in real time, the non-linearities, even if they are frequency dependent. In the analog feed forward technology, the non linearities are substracted from the input signal to amplify.
However, this solution has a very low power efficiency. According to the present state of the art the total power efficiency of an RF amplifier with analog feed forward linearisation is about 6% and a power amplifier has an efficiency of about 30% or more when it is not provided with this linearisation. Moreover, analog feed-forward is expensive and increases significantly the size of the device. The high cost comes from the fact that the corresponding circuits need calibration and tuning.
It is also known to use digital predistortion which has the advantage of a good efficiency. However, it has been observed that digital predistortions of amplifiers work correctly only on narrow and medium frequency bands.